Large-scale Plinian eruptions of the Colli Albani and the Campi Flegrei volcanoes : insights from textural and rheological studies

Abstract

In this study we investigated the mechanisms responsible for generating largescale Plinian eruptions. First, a detailed discussion of the state of the art of textural and rheological studies, explaining the main theories applied in the present work Then, eruptions with different compositions, and different viscosities, are chosen: phono-tephrites (Pozzolane Rosse) and tephri-phonites (Pozzolane Nere) from the Colli Albani volcano and trachytes (Agnano Monte Spina) from the Campi Flegrei caldera. All these magmas, despite their variability in chemistry and textural parameters, generate high explosive eruptions accompanied by caldera-forming events. Following an introductory part explaining the main features of the Colli Albani volcano, concerning geological setting, geochemical background and volcanic stratigraphy, we carried out combined investigation of textural, physical and chemical characteristics of scoria clasts (density, Vesicle Size Distributions, Crystal Size Distribution, viscosity), structural features of the deposit (grain-size, maximum clast size, composition) and eruption parameters (ejected volume, column height and discharge rate) of two of the largest ignimbrites of the Colli Albani volcano: the Pozzolane Rosse (RED, 457±4 ka) and the Pozzolane Nere (PNR, 407±4 ka) ignimbrites. Scoria clasts were analyzed in order to quantify textural features in scoriae from individual stratigraphic levels and to measure changes in vesicularity and crystallinity during the major shifts in eruptive intensity and style at Colli Albani volcano. Particular attention was given to the PNR fallout phase to reconstruct the main triggering factors and the first phases of the eruption. In order to evaluate the influence of the viscosity during the rise and the effect of crystal on the rheology itself, we carried out experiments with a concentric cylinder viscometer on the remelted PNR fallout. At high temperatures (1100-1225° C) the viscosity falls within the range 101.04 - 103.64 Pa s, while at low temperature (690-800° C) it ranges between 109.23 -1012 .15 Pa s. The effect of the crystals was evaluated by isothermal crystallization experiments in the region of subliquidus, in a range of T between 1193 and 1240° C at constant strain rate of 0.1 s-1. The experimental products were analyzed by the microprobe chemical analyses and observed under an optical microscope and scanning electron (SEM) for the textural analysis. The mineralogical assembly is constituted by the constant presence of leucite and iron oxides. The total percentage of the crystalline phases varies between about 6-22 % depending on the experimental temperature. On the basis of chemical and textural analysis, a model has been developed to predict the variation of the viscosity of the suspension liquid + crystals + bubbles to different water contents along the entire eruption of PNR. The relative influence of magma internal dynamic (viscosity changes) versus external factors (i,e, the vent diameter) responsible for the fall out-flow transition are discussed. We calculated the decompression rates, through the VNDs and determining some peculiar properties of the magma (e.g. diffusivity, density of the magma, surface tension). The decompression rates for the PNR are very fast and tend to decrease arising along the eruption. Subsequently, an implemented version of Conflow model (Mastin and Ghiorso 2000) called Confort2 was presented. The main modifications concern the liquid viscosity and crystal-bearing rheology determination, as well as the fragmentation criteria. The modified Fortran version Confort2 formulated can be applied to all natural magmas compositions, also including the peralkaline melts (pantelleritites) excluded in the original version. Furthermore, the crystal-bearing rheology was improved adding the effect of crystal shape on the rheology of natural magmatic suspensions and expanding the crystal content range in which it could originally be modeled (the original version of Confort works for crystal content up to 30%). This Fortran modified version Confort2 was applied on Pozzolane Nere eruption. Based on the investigations and on simulations, a model for the volcanological evolution of the Colli Albani ignimbrite eruptions is then presented. Then, we analyzed the textural characteristics and physical properties of the three major fallouts (A1, B1bottom and top, D1) and on the respective pyroclastic flows and surges (A2, B2, D2) of the Agnano Monte Spina eruption. The aim of this study was to relate the textural features of the deposit to intrinsic parameters of the magma during ascent along the conduit and to understand to what extent, if any, changes in the intrinsic properties of the magma may have affected the course of the eruption for the Agnano Monte Spina case. A combination of field, grain-size, density, Vesicle size distribution, Crystal size distribution, fine-ash morphologies and experimental viscosity investigations was carried out. The results of this study lead us to propose an eruptive scenario, taking particularly into account the role of volatiles into the different transitions between sustained and collapsed column. FTIR analyses show initial water content for A, B and D members of 1.77, 2.12 and 2.72 wt% (MIs, Arienzo, pers.comm.) and residual water content of 0.88-0.75 wt% (A1-A2), 0.72-0.60 wt% (B1-B2), 0.81-0.74 wt% (D1-D2). Isothermal viscosity measurements in the subliquidus region and their extrapolation at the eruptive temperature of 827°C and 945° C under different water contents, allow us to estimate the effect of crystals and vesicles on the AMS magmas. At estimated eruptive conditions and water content, viscosity of AMS magma reaches very high values compatible with the eruptive style of the eruption. Fragile fragmentation can be expected to occur. Values of viscosity of the AMS suspensions of the various members do not show variations greater than 0.6 logPa s as a function of crystal or bubble content. We can hence suggest that changes in viscosity during the eruption do not contribute significantly to variations in the eruption dynamics. We calculated the decompression rates which present a non-linear behavior. In general, the AMS eruption velocities are lower than the PNR ones (up to 120 m/s for D member), closer to the standard values for Plinian eruptions and mass discharges are comparable with those expected, ranging between 107-108 kg/s. A reconstruction of the eruptive sequence of the AMS eruption was then performed based on all textural, stratigraphic and rheological analyses. Finally, we presented a comparison between textural and rheological results of PNR and AMS eruptions, with special attention to the analyses of the transitions from sustained to collapsed column.